Variable pitch propeller systems typically incorporate a plurality of propeller blades mounted to a rotary hub driven by the aircraft's engine, with each propeller blade extending radially outwardly from the hub along the longitudinal axis of the blade. In order to permit pitch adjustment, each blade is mounted to the hub for pivotable movement about its longitudinal axis. The hub typically encloses a chamber within its interior wherein a pitch change actuation system is disposed which is operatively connected to the propeller blades. The actuation system functions to selectively change the pitch of the blades, therefore altering air resistance to the rotation of the blades to thereby control engine speed.
In most modern aircraft, the pitch change actuator is of the hydromechanical type wherein an output member, conventionally a hydraulic piston, is driven in response to blade loading and the pressure forces exerted by the hydraulic pitch change fluids exerted upon the opposite faces of the piston. Such hydromechanical pitch change actuation systems are well known in the art. For example, commonly assigned U.S. Pat. No. 4,523,891 to Schwartz and Duchesneau discloses a conventional pitch actuation system wherein each propeller blade is operatively connected to a piston which is driven by the pressure of a fluid which is selectively directed in response to a departure from desired engine speed against the opposite faces of the piston thereby causing a linear displacement of the piston and a resultant change in pitch of the blades operatively connected to the piston.
When the blade loading, which varies with blade pitch setting and other factors, is in balance with the net pressure force exerted by the pitch change fluids acting in opposition on the pitch change actuator piston, the blades remain at a their current pitch setting. If there exists a difference between the net pressure force exerted on the piston relative to the blade loading, the blades will be driven to a new pitch setting unless the net pressure force exerted on the actuator piston is adjusted to eliminate the imbalance relative to blade loading by increasing or decreasing as appropriate the pressure of the coarse pitch fluid and/or fine pitch fluid to maintain the actuator at a desired position, and thus the blades at the desired pitch setting, to maintain the desired engine speed. If it is desired to move the blades to a new pitch setting, for example so as to maintain a desired engine speed despite a change in blade loading due to other factors, the pressure of the coarse pitch fluid and/or that of the fine pitch fluid is selectively changed to appropriately adjust the net pressure force exerted upon the actuator piston relative to the blade loading so as to drive the blades to the desired new pitch setting.
The adjustments in fluid pressure are typically controlled by either a hydromechanical or electronic control system which monitors engine speed and selectively ports, by way of collateral apparatus, coarse and/or fine pitch change fluid at the proper fluid pressure to act upon the pitch change actuator piston whenever monitored engine speed departs from the desired engine speed setting. As a malfunction of the control system could result in an engine overspeed condition, a backup controller is typically provided in conventional blade pitch control systems to assume blade pitch control should a malfunction occur in the main pitch controller. For example, a pitch change control system having a main hydromechanical engine speed governor and a backup hydromechanical engine speed governor is disclosed in commonly assigned U.S. Pat. No. 4,588,354. As disclosed therein, a least selector valve is provided which continuously compares the pressure of the pitch change fluid supplied through the main governor with the pressure of the pitch change fluid supplied through the backup governor. The least selector valve passes the lowest magnitude pressure signal to a piston actuator which sets the blade pitch in accordance with the magnitude of the passes pressure signal, whereby the governor providing the lowest pressure signal controls blade pitch independently of the other governor.
Blade pitch control systems having a constant speed controller, an overspeed controller, a computer for controlling operation of at least the constant speed controller, and preferably also the overspeed controller are also known in the art. For example, in European Patent Application Publication 0 311 277, there is disclosed a pitch control system having two electrohydraulic servo-valve controllers, one for controlling constant speed operation and the other for controlling overspeed operation, a microprocessor for controlling each of the electrohydraulic servo-valves, and a feathering valve for overriding the constant speed servo-valve by supplying high pressure fluid to the pitch actuator to drive the blades to feather in the event of a failure of the constant speed servo-valve preventing feathering of the blades.